Niobium alkoxides and tantalum alkoxides are useful as starting materials for oxides including niobium oxides or tantalum oxides usable as materials for dielectrics. Such an alkoxide is calcined to form the powdery oxide; the alkoxide is hydrolyzed and then the hydrolyzate is calcined to form an oxide powder; the alkoxide is hydrolyzed, a coating film is formed from the hydrolyzate by the sol-gel method and then the film is burned to obtain an oxide film; or the alkoxide is used for forming an oxide film by MOCVD method. The composition of the oxides varies in a wide range from a single oxide to a composite oxides comprising plural components. When such an oxide is used as an electronic material for dielectrics, it is required to reduce the quantity of impurities such as transition metals typified by iron, alkali metals, e.g. sodium, alkaline earth metals, e.g. calcium, as well as uranium and thorium as far as possible. These impurity elements cause various disorders such as the reduction in the durability of electronic devices, increase in the leakage of current, and software errors. Recently, tantalum ethoxide is used for producing a tantalum oxide film which is a high dielectric constant paraelectric substance as a capacitor for DRAM (Dynamic Random Access Read/Write Memory) by MOCVD method. The use of niobium alkoxide or tantalum alkoxide as a starting material for a non-volatile memory by using a thin ferroelectric film of SrBi.sub.2 Ta.sub.2 O.sub.9 or SrBi.sub.2 Nb.sub.2 O.sub.9 will be further developed.
However, niobium alkoxides and tantalum alkoxides usually contain a very small amount of elements such as iron, calcium, sodium and uranium and compounds of them (hereinafter referred to as "impurities"). These impurities were incorporated thereinto from starting materials for the alkoxides or materials for the reactors; from the atmosphere surrounding the apparatuses, starting materials, intermediate materials and products; or from additives used for the synthesis or purification.
For removing the impurities from a niobium alkoxide or tantalum alkoxide, fractional distillation is considered to be easy. However, since the niobium alkoxide or tantalum alkoxide having a low vapor pressure necessitates the vacuum distillation, the separation and purification conducted taking the advantage of the difference in the vapor pressure between the alkoxide and the impurities is very difficult. Another disadvantage of this method is that the vapor pressure of the impurity is close to that of the niobium alkoxide or tantalum alkoxide, or that they form a double alkoxide to make the vapor pressures of them further closer. Still another disadvantage is that this method necessitates a complicated, expensive apparatus.
Japanese Patent Publication No. 58194/1989 discloses a process for purifying alkoxides of Al, Ga, In, Y, Si, Ti, Zr, etc. in the form of a solution and containing at least one of Ti, Fe, Cu, Si, Na and U impurities by hydrolyzing 0.1 to 50% of the metal alkoxides under stirring to form a solid reaction product and then separating the reaction product from the unreacted metal alkoxides by the distillation to recover the metal alkoxides. However, the specification is silent on niobium alkoxides and tantalum alkoxides which are pentavalent alkoxides. As for the degree of the purification, although the specification discloses that, for example, Fe content of aluminum isopropoxide as the starting material can be reduced from 750 ppm to 1 or &lt;1 ppm by the purification, the specification is silent on the high degree of the purification intended by the inventors of the present invention which is about 0.01 to 0.001 ppm. In addition, the specification of Japanese Patent Publication No. 58194/1989 is silent on the removal of calcium and strontium.
The distillation is insufficient for the purification of niobium alkoxides and tantalum alkoxides having an ordinary purity to obtain a high purity. When impurities contained in niobium alkoxides and tantalum alkoxides in even only a very small amount of an order of ppb are not allowed, it is necessary to purify these alkoxides in the final stage of the production thereof because they are possibly contaminated in the course of the production of them even if the starting materials and additives are very carefully handled. However, efficient purification process which can be employed for this purpose on an industrial scale has not been developed yet.
The object of the present invention is to provide a process for efficiently removing a very small amount of impurities such as Fe, Ca, Sr, Na and U from niobium alkoxides and tantalum alkoxides to reduce the impurity level to an order of ppb.